The invention is located in the field of converters of optical signals in the return to zero (RZ) format into optical signals in the non-return to zero (NRZ) format.
It is well known that it is preferable to transmit binary digitalxe2x80x94for example telecommunications-signals, in an NRZ format rather than in an RZ format, since NRZ format signals consume less band width, for a same information flow, than RZ format signals.
However some so-called 3R regeneration processes standing for xe2x80x9cReamplification, Reshaping, Retimingxe2x80x9d require for example a first conversion from NRZ format to RZ format then a return to NRZ format for retransmission after regeneration. The result is that RZ to NRZ converters are already known.
In most of the known RZ-NRZ converters, broadening pulses representing a 1, of half a bit time in length, in order to convert them into pulses having a length of one bit time is obtained in two phases. Firstly the signal is carried into a dispersive fibre. Given the slight variations in wavelength present in the signal and the different propagation speed in a fibre dispersing these wavelengths, the pulses representing levels 1 are broadened. Then the broadening of the fibre output signal is increased by adding to this first signal a second signal obtained from the initial RZ signal by wavelength conversion, passing through a dispersive fibre and a delay of half a bit time. After the power addition of these two signals of different wavelengths, the resulting signal must pass into a second wavelength converter so that the NRZ output signal comprises only a single wavelength. In these conditions the pulses representing 1""s of the initial signal are broadened to a value approximately equal to one bit time in the signal resulting from the addition of the first and second signals.
It can thus be seen that the known method requires an RZ signal distribution occurring on two parallel channels, a wavelength conversion of the signal to be converted on one of the channels, a delay of a half bit time on one of the channels relative to the other channel, an addition of signals present at the output of each of the two channels, and lastly a wavelength reconversion of the signal resulting from this addition so as to have a single wavelength. It should be noted that the wavelength conversion on one of the two channels is necessary to avoid destructive interference between the two signals, in a part where they overlap in time during the addition of the two signals.
The purpose of the invention is to obtain an NRZ signal containing the same information as an initial RZ signal, whose symbols representing xe2x80x9c1xe2x80x9d have a stable power level, well defined rising edges and falling edges. According to the method used to embody the invention, we obtain a signal converted to the same wavelength as the initial NRZ signal or on the contrary a wavelength converted signal.
The idea underpinning the invention is to use an optical bistable means with hysteresis. Such an optical means has an output power, which varies with the input power in the following way. When the input power increases, the output power of the optical bistable means with hysteresis increases at first slowly then changes level very rapidly with the input power to set itself at a much higher high level as soon as the input power crosses in an upward direction a first threshold. When the output signal of the optical bistable means with hysteresis is at the high level, if the input power reduces, the output power of the optical bistable means with hysteresis starts by reducing slowly then reduces very rapidly with the reduction in input power to set itself at a much lower low level as soon as the input power crosses in a downward direction a second threshold. The second threshold is below the first.
The use according to the invention of this property of the optical bistable means will now be explained.
From the initial RZ signal to be converted is made a control signal of the optical bistable means. This signal is then input into the optical bistable means. This control signal has the following properties. For a transition from a value 0, i.e. from a low logic level lasting more than one bit time, to a value 1 of the initial RZ signal to be converted, the control signal passes from a level below the first threshold S1 to a level above the first threshold. In the same way the output signal of the optical bistable means with hysteresis passes from a low level to a high level. When the initial RZ signal has a sequence of 1""s, one after the other, the input signal power level may fluctuate due to the returns to 0 for less than one bit time of the initial signal, but this input signal power level must remain above the second threshold in such a way that the output signal retains the high level. If on the other hand the initial RZ signal returns to 0 and remains there for a time longer than one bit time, which means that the sequence of 1""s is finished, then the control signal must have an input level which is set at a level below the second threshold of the optical bistable means with hysteresis then which will remain below the first threshold, so long as the RZ signal remains at the logic level 0. To sum up the invention relates to a device for converting a binary digital optical signal in the RZ format having a flow rate D, and therefore a bit time 1/D, into a binary digital optical signal in the NRZ format with the same flow rate characterised in that it comprises:
an optical bistable means having an input and an output, an output logic level of this optical bistable means passing from a low level to a high level when the power level present at input passes from a level below a first threshold to a level above this first threshold and passing from a high level to a low level when the power level present at input passes from a level above a second threshold to a level below this second threshold, the second threshold being below the first threshold, the output of this optical bistable means carrying the NRZ format signal;
a means for converting the binary digital optical signal in the RZ format into a control signal of an output logic level of the optical bistable means, this conversion means having an input and an output, this means receiving at its input, the RZ signal to be converted and delivering at its output said control signal, said output being coupled to the input of the optical bistable means, the conversion means producing a signal having a level above the first threshold when a signal present at the input of this circuit passes from a low logic level present for more than one bit time to a high logic level, remains at a level above the second threshold so long as the input signal does not return for more than one bit time to a low logic level, and producing a signal having a level below the second threshold when a signal present at the input of this means passes from a high logic level to a low logic level and remains there for more than one bit time.
Examples of embodiments of means of converting an RZ signal into a control signal will be given later.
The bistable means may be a passive means, like for example a microcavity incorporating a saturable absorbent material. In this case the NRZ output signal may or may not be converted in wavelength as the designer chooses. The control signal must have a wavlength whose value is set within one of the windows of transparency of the microcavity, i.e. which is resonant in the microcavity.
The bistable means may also be an active means, like for example a Q switch laser. In this case the NRZ output signal will be converted in wavelength and will be at the wavelength of the Q switch laser.